In the drilling of deep wells such as oil and gas wells, it is common practise to drill utilizing the rotary drilling method. A suitably constructed derrick suspends the block and hook arrangement, together with a swivel, drill pipe, drill collars, other suitable drilling tools, for example reamers, shock tools, etc. with a drill bit being located at the extreme bottom end of this assembly which is commonly called the drill string.
The drill string is rotated from the surface by the kelly which is rotated by a rotary table. During the course of the drilling operation, drilling fluid, often called drilling mud, is pumped downwardly through the hollow drill string. This drilling mud is pumped by relatively large capacity mud pumps. At the drill bit this mud cleans the rolling cones of the drill bit, removes or clears away the rock chips from the cutting surface and lifts and carries such rock chips upwardly along the well bore to the surface.
In more recent years, around 1948, the openings in the drill bit allowing escape of drilling mud were equipped with jets to provide a high velocity fluid flow near the bit. The result of this was that the penetration rate or effectiveness of the drilling increased dramatically. As a result of this almost all drill bits presently used are equipped with jets thereby to take advantage of this increased efficiency. It is worthwhile to note that between 45-65% of all hydraulic power output from the mud pump is being used to accelerate the drilling fluid or mud in the drill bit jet with this high velocity flow energy ultimately being partially converted to pressure energy with the chips being lifted upwardly from the bottom of the hole and carried to the surface as previously described.
As is well known in the art, a rock bit drills by forming successive small craters in the rock face as it is contacted by the individual bit teeth. Once the bit tooth has formed a crater, the next problem is the removal of the chips from the crater. As is well known in the art, depending upon the type of formation being drilled, and the shape of the crater thus produced, certain crater types require much more assistance from the drilling fluid to effect proper chip removal than do other types of craters. For a further discussion of this see "Full Scale Laboratory Drilling Tests" by Terra-Tek Inc., performed under contract Ey-76c-024098 for the U.S. Department of Energy.
The effect of drill bit weight on penetration rate is also well known. If adequate cleaning of the rock chips from the rock face is effected, doubling of the bit weight will double the penetration rate, i.e. the penetration rate will be directly proportional to the bit weight. However, if inadequate cleaning takes place, further increases in bit weight will not cause corresponding increases in drilling rate owing to the fact that formation chips which are not cleared away are being reground thus wasting energy. If this situation occurs, one solution is to increase the pressure of the drilling fluid thereby hopefully to clear away the formation chips in which event a further increase in bit weight will cause a corresponding increase in drilling rate. Again, at this increased drilling rate, a situation can again be reached wherein inadequate cleaning is taking place at the rock face and further increases in bit weight will not significantly affect the drilling rate and, again, the only solution here is to again increase the drilling fluid pumping pressure thereby hopefully to properly clear the formation chips from the rock face to avoid regrinding of same. Those skilled in the art will appreciate that bit weight and drilling fluid pressure must be increased in conjunction with one another. An increase in drilling fluid pressure will not, in itself, usually effect any change in drilling rate in harder formations; fluid pressure and drill bit weight must be varied in conjunction with one another to achieve the most efficient result. For a further discussion of the effect of rotary drilling hydraulics on penetration rate, reference may be had to standard texts on the subject.
It should also be noted that in softer formations, the bit weight that can be used effectively is limited by the amount of fluid cleaning available below the bit. In very soft formations the hydraulic action of the drilling fluid may do a significant amount of the removal work.
In an effort to increase the drilling rate, the prior art has provided vibrating devices known as mud hammers which cause a striker hammer to repeatedly apply sharp blows to an anvil, which sharp blows are transmitted through the drill bit to the teeth of the rolling cones. This has been found to increase the drilling rate significantly; the disadvantage however is that the bit life is significantly reduced. In a deep well, it is well known that it takes a considerable length of time to remove and replace a worn out bit and hence in using this type of conventional mud hammer equipment the increased drilling rate made possible is offset to a significant degree by the reduction in bit life.
One proposal for cyclically interrupting flow through a drill stem is disclosed in U.S. Pat. No. 2,780,438 issued Feb. 5, 1957. This patent proposes the use of a rotary valve member actuated by a spiral rotary valve actuator. Axially disposed co-operating passages are provided in the valve structure and thrust bearings take up axially oriented loads on the rotary valve member. Disadvantages of this proposal include the fact that the axially oriented passages are prone to blockage by debris. The high shock forces on the rotary valve member would tend to rapidly destroy the thrust bearings supporting the rotary valve. The overall arrangement would be very inefficient in providing fluctuating forces on the drill bit. The free telescoping movement of the housing above the rotary valve would destroy most of the desired water hammer effect and would appear to eliminate most of the pressure drop below the bit considering that the apparatus is acting in a closed system.
Another prior art flow pulsing arrangement is shown in the Zublin U.S. Pat. No. 2,743,083 issued Apr. 24, 1956. This patent shows several embodiments of an invention. In all of these embodiments, however, the arrangement is such that pressure pulses above the rotor and consequent pressure drops below the rotor act on almost the whole projected area of the rotor. High axial forces on the rotor bearings result thus materially shortening the bearing life. Furthermore, the valving arrangements provided are prone to jamming due to debris in the drilling fluid and if sufficient clearance is provided to alleviate jamming problems the structural configuration of the valve makes it difficult to achieve a meaningful level of pressure build-up.
My above-noted copending U.S. pat. applications Ser. Nos. 008963 and 626,121 (disclosures of which are incorporated herein by reference hereto) disclose improved forms of flow pulsing apparatus including a rotor having blades which is adapted to rotate in response to the flow of drilling fluid through the tool housing. A rotary valve forms part of the rotor and alternately restricts and opens the fluid flow passages thereby to create cyclical pressure variations. The flow passages comprise radially arranged port means in a valve section of the housing with the rotary valve means being arranged to rotate in close co-operating relationship to the port means to alternately open and close the radial ports during rotation.
Because of the fact that the drilling fluid typically contains a substantial portion of gritty material of varying size as well as other forms of debris such as sawdust and wood chips, and since it is not practical to attempt to screen or filter all of this material out of the drilling fluid, all of the above-described rotary valve arrangements are prone to jamming due to debris binding in the valve surfaces. Accordingly, there is a requirement that a degree of clearance be maintained between the valve surfaces and in my above-noted copending applications various improvements have been incorporated thereby to allow the radial clearances between the valving surfaces to be kept as small as possible while at the same time avoiding jamming under ordinary circumstances. It should be kept in mind, of course, that in order to achieve the maximum water hammer effect, the clearances should be kept as small as possible thereby to achieve the maximum possible conversion of the flow energy of the drilling fluid into dynamic pressure energy to produce the optimum water hammer effect. The structures described in my copending applications above require a minimum radial clearance in order to avoid binding and jamming. Hence, it can readily be seen that the total "leakage" area when the valve is "closed" will be equal to the clearance dimension multiplied by the total distance around the valve ports. Since there is a need to keep the total leakage area relatively small, it follows that the total distance around the valve ports must be kept reasonably small as well, resulting in much smaller than optimum port holes which in turn restrict the flow unduly even when the valve is fully open thus creating a substantial pressure drop across the open valve. This restriction of the flow through the fully open valve reduces the overall operating efficiency of the system for reasons which will be readily apparent to those skilled in the art.
Another disadvantage associated with rotary valve flow pulsating arrangements is that the timing or frequency of the fluctuation is strictly governed by the angular velocity of the rotor. Another disadvantage is that the shape of the pressure pulse curve cannot be easily varied or changed to better suit conditions.